Re: [patch] Make std::tr1::shared_ptr thread-safe.

[Jonathan Wakely <cow at compsoc dot man dot ac dot uk>]

On Wed, Mar 30, 2005 at 10:23:21AM -0800, Ulrich Drepper wrote:

> Paolo Carlini wrote:
> 
> >Then, readd to the library implementations of exchange_and_add for the
> >platforms that can implement it, otherwise fall back to the stop-gap
> >solution (some configury bits needed).
> 
> Using exchange_and_add is dangerous, it should not be used.
> 
> Granted, it will hurt only broken programs, but only those will use an
> object with zero use anyway.
> 
> The problem is that is two thread simultaneously call lock, one suddenly
> succeeds since one of the exchange_and_add comes first.  The result is
> the first thread will throw an exception and destroy the object while
> the second thinks all is fine and continues its work with a broken pointer.
> 
> The copmare_and_exchange I used is the correct solution.

This is what I have in my working copy (where I've resurrected
compare_and_swap) but I make no claims that it's correct:

  void
  add_ref_lock()
  {
    _Atomic_word __tmp = _M_use_count;
    while (__tmp)
    {
      if (__gnu_cxx::__compare_and_swap(&_M_use_count, __tmp, __tmp+1))
        return;
      __tmp = _M_use_count;
    }
    __throw_bad_weak_ptr();
  }